The Cell Biochemistry Program has focused on biochemically characterizing osteogenic cells, including marrow stromal fibroblasts (MSFs), a heterogeneous population of clonogenic cells that give rise to osteoblasts/chondrocytes, adipocytes and hematopoiesis supportive stroma. Studies were designed to test the hypothesis that products produced by non-adherent (hematopoietic) cells in bone marrow cultures are responsible for stimulating the proliferation of CFU-F (colony forming unit- fibroblastic) to form MSF colonies. By using blocking antibodies in combination with medium conditioned by non-adherent marrow cells, it was found that PDGF, TGF-beta, bFGF and EGF are required to stimulate colony formation. However, it was also found that MSFs from different animal species have different growth factor requirements. Mouse MSFs require both serum and products synthesized by non-adherent cells and that colony formation can be partially stimulated in part by bFGF alone, whereas human MSFs require only serum, and no single factor stimulates colony formation. In addition to determining the growth factor requirements, the osteogenic capacity of individual clones of human MSFs was determined for the first time by using a newly developed in vivo transplantation system. It was found that 54% of the clones studied supported bone formation, but only half of these clones supported bone formation and hematopoiesis. The differences between non-bone forming clones, clones that form bone and those that form bone and support hematopoiesis are currently under investigation.